Fuel injectors for internal combustion engines commonly used solenoid-operated valves to meter fuel under pressure either upstream of a manifold-type distribution system or on an individual cylinder basis at a point near the intake valve. The former arrangement is commonly called "throttle body injection" and the latter is commonly called "multipoint injection".
More recently, it has been discovered that the fuel metering function and an atomizing function can be achieved using an acoustically resonant structure that is periodically excited with an alternating current excitation signal. Although such structure may take various forms, it may be generally described as comprising the combination of a mechanical device, such as a catenoidal horn-shaped injector body, and an electrical device such as a piezoelectric crystal or an arrangement of several such crystals. One combination pertinent to the invention described herein comprises a catenoidal horn having a ball check valve in the fuel flow path near the small tip of the horn and a pair of electrically parallel connected piezoelectric crystals mechanically abutting the large end of the horn. When the crystals are excited by an alternating current pulse of controlled frequency and amplitude, the horn is set into resonant vibration to unseat the ball and to permit a metered quantity of fuel to flow to the combustion chamber or chambers.
The successful use of an acoustic fuel injector requires the ability to precisely control the injected fuel quantity under varying operating conditions. Such control is, in great measure, affected by the degree to which the frequency of the excitation signal matches the mechanically resonant frequency of the acoustic structure; i.e., even a small mismatch results in decreased vibration amplitude at the tip of the horn where metering and atomization take place. This is a difficult match to maintain because, as previously described, the resonant structure includes both electrical and mechanical components. Moreover, the resonant frequency of the structure is not constant; rather, it is known to vary significantly with temperature, load, and contamination level. Unless the frequency of the excitation signal can be made to follow such variations in mechanical resonant frequency, precise fuel metering is not possible.
The ability to generate an excitation signal of controlled frequency and amplitude is at least in part dependent upon the stability of the DC voltage which is available to the excitation signal oscillator-generator. As a result, there are numerous applications, particularly in the automotive and vehicular fields, for a DC-to-DC converter which operates to provide a highly stabilized output voltage despite substantial variations in the supply voltage furnished by, for example, a 12 V automotive battery.
In the fuel injection system application, as well as other applications, it is also advantageous to provide a reliable start-up function for the converter oscillator and to minimize the number of required components for production economy.
An oscillator circuit with reliable start-up characteristics is shown in FIG. 2.43 of "Design of Solid State Power Supplies", Second Edition by E. R. Hnatek, van Nostrand Reinhold Company, p. 78. In that circuit, the primary winding of an output transformer in the oscillator output circuit is coupled to a tertiary winding in the oscillator input circuit to provide positive feedback. FIGS. 2.44 and 2.45 on Page 80 of the same publication illustrate converter circuits having a feedback connection from the output circuit to the inputs of the oscillator for regulation purposes. However, these circuits are characterized by a large number of circuit components with a corresponding lack of product economy.
A related aspect of prior art fuel injection systems is the interrelationship of component power handling capability versus characteristic response time. These circuits tended to trade off economy for enhanced response.